1. Field of the Invention
The present invention relates to an electronic part mounting method, a semiconductor module, and a semiconductor device.
2. Description of the Related Art
Usually, a high-frequency power amplifier is incorporated in a portable communication unit represented by a car phone or a cellular phone based on the PDC (Personal Digital Cellular) system, or a cellular phone based on the PHS (Personal Handy-phone System). As well known, the high-frequency power amplifier is constituted as a semiconductor module, and it is a multistage amplifier comprising a plurality of amplifiers interconnected in multiple stages.
Such a high-frequency power amplifier is formed on one principal surface of a semiconductor chip, and the semiconductor chip including the amplifier is mounted on one principal surface of a wiring board. Electrodes formed on the one principal surface of the semiconductor chip and electrodes formed on the one principal surface of the wiring board are connected to each other by conductive wires.
More specifically, the high-frequency power amplifier comprises, for example, a plurality of field effect transistors electrically connected in parallel. A gate terminal serving as an input portion of the high-frequency power amplifier is electrically connected to a chip-side input electrode formed on the one principal surface of the semiconductor chip.
On the other hand, a drain terminal serving as an output portion of the high-frequency power amplifier is electrically connected to a chip-side output electrode formed on the one principal surface of the semiconductor chip. The chip-side input electrode is arranged at one side of the semiconductor chip, and the chip-side output electrode is arranged at another side of the semiconductor chip, which is positioned in opposite relation to the one side.
A source terminal of the high-frequency power amplifier is electrically connected to a rear-side electrode formed on another surface (i.e., a rear surface) of the semiconductor chip, which is positioned in opposite relation to the one principal surface of the semiconductor chip. The rear electrode is fixedly held at a reference potential. The chip-side input electrode is electrically connected, by an input wire, to a board-side input electrode formed on the one principal surface of the wiring board in facing relation to the one side of the semiconductor chip. The chip-side output electrode is electrically connected, by an output wire, to a board-side output electrode formed on the one principal surface of the wiring board in facing relation to the other side of the semiconductor chip.
One practical semiconductor module is described in “Hitachi Hyoron”, No. 4, 1993, pp. 12-26. published Apr. 25, 1993 by Hitachi Hyoron Co. In this semiconductor module (i.e., a MOS power module for the high-frequency power amplifier), power MOSFET's are incorporated in three stages to increase an output.
Non-Patent Reference 1 further discusses various types of semiconductor modules in the packaged form. The semiconductor module assembled in a cellular phone employs a metallic cover and surface mounting for the purpose of size reduction.
FIG. 15 shows the structure of a known semiconductor module employing a metallic cover (hereinafter referred to as a “cap”) and the surface mounting. A glass-ceramic substrate 202 is fixed to a principal surface (upper surface) of a heat radiating flange 201 in the form of a rectangular plate by using a solder (not shown).
Active parts, such as power MOSFET's (not shown), and passive parts, such as resistances and capacitors (not shown), are mounted on a principal surface (upper surface) of the substrate 202. The active parts, such as the power MOSFET's, are connected to external terminals by wire bonding.
Further, a cap 203 is attached to the heat radiating flange 201 so as to cover the principal surface of the glass-ceramic substrate 202. Openings are formed in one side surface of the cap 203, and leads 204 having inner ends fixed to the glass-ceramic substrate 202 are led out through the openings. Fins 205 for face fixing are each disposed to project in the form of one step (i.e., an L-like bent shape) outward from a side edge of the heat radiating flange 201.
The fins 205 for face fixing not only serve to transmit heat to a chassis (not shown) on which a high-frequency power module 206 is mounted, but also function as a ground pin. The heat radiating flange 201 is electrically connected to ground wiring on the principal surface of the glass-ceramic substrate 202 through a conductor filled in a through hole formed in the glass-ceramic substrate 202.
JP-A-10-50926 discloses a heat radiating module in which a heat-generating circuit part is placed in a recess formed in a circuit board, and solder bumps of the circuit part are soldered to land electrodes of a circuit board. Further, the thus-obtained circuit board is mounted on a mother circuit board through a heat conductive member, and terminal electrodes provided on a side surface of the circuit board are soldered to land electrodes on the mother circuit board, thereby transmitting the heat generated from the circuit part to the mother circuit board for the purpose of heat radiation.
Meanwhile, in a non-insulation semiconductor device as one of power semiconductor devices for use in inverters, etc., a member used for fixing a semiconductor element serves also as one electrode of a semiconductor device. For example, in a semiconductor device including a power transistor mounted on a fixing member (made of, e.g., a copper—cuprous oxide composite material) by using an Sn—Pb brazing paste, the fixing member (base material) serves also as a collector electrode of the power transistor.
In actual operation, a collector current of several amperes or more flows, whereupon a transistor chip generates heat. To avoid the generated heat from causing instability of characteristics and shortening of the life, the base material is required to have superior heat dissipation and to ensure reliability in a brazed portion. Then, ensuring reliability in the brazed portion requires matching in thermal expansion rate between the semiconductor element and the fixing member.
Also in an insulation semiconductor device, from the viewpoint of realizing the safe and stable operation of the semiconductor element, it is required to efficiently dissipate the heat generated during the operation of the semiconductor device to the outside of a package, and to ensure reliability in the brazed portion.
To clear those conditions, JP-A-8-11503) discloses a semiconductor current controller in which an assembly obtained by mounting a Si chip on a Cu-coated AlN board is integrally brazed to a support member made of Mo by using a solder. With this disclosed technique, since the Cu-coated AlN board is soldered to the Mo support member having a thermal expansion rate (5.1 ppm/° C.) comparable to that of the Cu-coated AlN board, a soldered portion between both the members has superior reliability.
Patent Reference 3 (JP-B-7-26174) discloses a semiconductor module device in which an assembly obtained by mounting a thyristor chip on an alumina board is mounted to a support member made of a composite material in which SiC ceramic powder is dispersed in Al or an Al alloy. With this disclosed technique, since the alumina board is mounted to the support member made of the Al/SiC composite material having a thermal expansion rate (2 to 13 ppm/° C.) comparable to that (7.5 ppm/° C.) of the alumina board, a bonded portion between both the members has superior reliability.